A&A 472, 311–319 (2007) Astronomy DOI: 10.1051/0004-6361:20066927 & c ESO 2007 Astrophysics Light curves and colours of the faint Uranian irregular satellites Sycorax, Prospero, Stephano, Setebos, and Trinculo, M. Maris1, G. Carraro2,3, and M. G. Parisi4,5,6 1 INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy e-mail: [email protected] 2 Dipartimento di Astronomia, Università di Padova, Vicolo Osservatorio 2, 35122 Padova, Italy e-mail: [email protected] 3 Andes Prize Fellow, Universidad de Chile and Yale University 4 Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile e-mail: [email protected] 5 Member of the Centro de Astrofisica, Fondo de Investigacion Avanzado en Areas Prioritarias (FONDAP), Chile 6 Member of the Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), Argentina Received 13 December 2006 / Accepted 17 April 2007 ABSTRACT Context. After the work of Gladman et al. (1998, Nature, 392, 897), it is now assessed that many irregular satellites are orbiting around Uranus. Aims. Despite many studies performed in past years, very little is know about the light-curves of these objects and inconsistencies are present between colours derived by different authors. This situation motivated our effort to improve both the knowledge of colours and light curves. Methods. We present and discuss, the time series observations of Sycorax, Prospero, Stephano, Setebos, and Trinculo, five faint, irregular satellites of Uranus, which were carried out at VLT, ESO Paranal (Chile) on the nights between 29 and 30 July, 2005 and 25 and 30 November, 2005. Results. We derive light curves for Sycorax and Prospero and colours for all of these these bodies. Conclusions. For Sycorax we obtain colours B−V = 0.839 ± 0.014, V −R = 0.531 ± 0.005, and a light-curve which is suggestive of a periodical variation with period ≈3.6 h and amplitude ≈0.067 ± 0.004 mag. The periods and colours we derive also for Sycorax are in agreement with our previous determination in 1999 using NTT. We also derive a light-curve for Prospero which suggests an amplitude of about 0.2 mag and a periodicity of about 4 h. However, the sparseness of our data, prevents a more precise characterization of the light-curves, and we can not determine whether they are one-peaked or two-peaked. Hence, these periods and amplitudes have to be considered preliminary estimates. As for Setebos, Stephano, and Trinculo the present data do not allow us to derive any unambiguous periodicity, despite the fact that Setebos displays a significant variability with amplitude about as large as that of Prospero. Colours for Prospero, Setebos, Stephano, and Trinculo are in marginal agreement with the literature. Key words. planets and satellites: general – methods: observational – methods: data analysis – methods: statistical – methods: numerical 1. Introduction temporary capture into a permanent one requires a source of dissipation of orbital energy and that particles remain inside the In recent years many irregular satellites have been discovered Hill sphere long enough for the capture to be effective (Pollack around Uranus (Gladman et al. 1998; Kavelaars et al. 2004; et al. 1979). Otherwise, the trapped object will escape within at Gladman et al. 2000; Sheppard et al. 2005). Irregular satellites most a few hundred orbits (Byl & Ovenden 1975; Heppenheimer / are those planetary satellites on highly elliptic and or highly 1975; Heppenheimer & Porco 1977; Pollack et al. 1979). During inclined (even retrograde) orbits with a large semi-major axis. the planet formation epoch several mechanisms may have op- These objects cannot have formed by circumplanetary accre- erated, some of which have the potential to be active even tion like the regular satellites, but they are likely products of after this early epoch. They fall mainly into few categories: captures from heliocentric orbits, probably in association with collisional interactions (Colombo & Franklin 1971), pull-down the planet formation itself (Greenberg 1976; Morrison & Burns capture (Heppenheimer & Porco 1977), gas drag (Pollack et al. 1976; Morrison et al. 1977; Jewitt & Sheppard 2005). It is 1979), four-bodies interactions in the reference frame of the the possible for an object circling about the Sun to be temporar- Sun-Planet system either between the captured body and a large ily trapped by a planet (Heppenheimer 1975; Greenberg 1976; regular satellite of the planet (Tsui 2000), or between the two Morrison & Burns 1976, to cite only some). But to turn a components of a binary object leading to an exchange reaction where one of the components of the binary is captured and the Based on observations with the ESO Very Large Telescope + other is ejected from the system (Agnor & Hamilton 2006). FORS2 at the Paranal Observatory, Chile, under program 075.C-0023. Table 1 is only available in electronic form at Collisional capture, the so called break-up process, leads to http://www.aanda.org the formation of dynamical groupings. The resulting fragments Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20066927 312 M. Maris et al.: VLT Photometry of 5 Uranian Irregulars of the progenitor body after a break-up will form a population The paper is organised as follows: Sect. 2 describes obser- of irregular satellites expected to have similar composition, i.e., vations and data reduction, light-curves are discussed in Sect. 3, similar colours, and irregular surfaces. Large temporal variations while Sect. 4 presents the satellites’ colours. The conclusions are in the brightness of irregular satellites are expected from rotat- reported in Sect. 5. ing bodies of highly elongated shapes and/or irregular surfaces consistent with a collision fragment origin. 2. Observations and data reduction Gas drag is expected to occur in the environment of the protoplanetary nebula (Byl & Ovenden 1975; Horedt 1976; We observed the irregular satellites Sycorax, Prospero, Heppenheimer & Porco 1977; Pollack et al. 1979) and it may be Stephano, Trinculo and Setebos with the FORS2 camera that the origin of dynamical families of fragments. In this case (Appenzeller et al. 1998) at the focus of VLT Antu telescope fragments would be produced by the hydrodynamical breaking in Paranal, Chile, in the two consecutive nights of July 28 and of the intruding body into smaller chunks in the case that they 29, 2005. exceed the tensile strength of the entering body (Pollack et al. We used the standard FORS2 B, V, R, I filters1, which are 1979). The gravitational attraction of fragments prevents them very close to the Bessel system. In particular, the effective wave- from escaping, in general the hydrodynamical pull being not length, λeff,andFWHM,∆λ, for the filters reported by ESO are larger than self-gravity, but a small impact of a ∼1 km size ob- λeff,B = 0.429 µm, ∆λB = 0.0880 µmforB; λeff,V = 0.554 µm, ject, likely common in the nebula environment, would be suffi- ∆λV = 0.1115 µmforV; λeff,R = 0.655 µm, ∆λR = 0.1650 µm cient to disperse them without introducing a further fragmenta- for R;andλeff,I = 0.768 µm, ∆λI = 0.1380 µmforI. Stephano tion (Pollack et al. 1979). A specific prediction of this scenario and Trinculo were observed in the same frames so that we is the production of fragments with a more regular/round sur- observe five objects with just four sequences. For this reason face than in the break-up process, leading to a light-curve with Stephano R1, R2, ..., V1, V2, ..., frames correspond to Trinculo low amplitude variations (Pollack et al. 1979). R1, R2, ..., V1, V2, ..., frames. Each object was observed in On the contrary, if pull-down capture, four-bodies interac- consecutive sequences of frames. After the end of the sequence tions or exchange reactions are the dominant causes of formation for a given object the telescope switched to the sequence of an- of the irregular satellites, each object would be the result of an other object. Ideally, colours would have to be calculated by independent capture event. In this case, no obvious correlation combining magnitudes from frames in the same sequence, in or- between dynamical properties, colours, and light-curves would der to limit the rotational effects. For each sequence, pointing be expected. of the telescope and orientation of the camera were kept fixed. Casting light on these scenarios, colours, and light curves During the acquisition of each frame the telescope was tracked are very important, since they would allow one to discriminate at the same rate the target, while the telescope was reset at the between collisional or non-collisional origin for irregular satel- default pointing at the beginning of each frame in the sequence. lites. Theories of irregular satellite capture have lacked many However, given the slow proper motion and the short exposures, constraints. However, the rapidly-growing number of known ir- the effect of differential tracking on background stars was negli- regular satellites is now providing new insights on the processes gible, background stars do not appear elongated. Interruptions of planet formation and satellite capture. due to a ToO, mid-night calibrations, and some minor prob- A possible origin of the large obliquity of Uranus is a gi- lem prevent us from keeping the same sequences both nights. ant impact event between the planet and an Earth-sized plan- Both nights were photometric, with average seeing ≈ 1.1 arcsec. etesimal, occurred at the end of the epoch of accretion (Slattery FORS2 is equipped with a mosaic of two 2k × 4k MIT CCDs (pixel size of 15 × 15 micron) with a pixel scale, with the de- et al. 1992; Parisi & Brunini 1997). The dynamical and physi- cal properties of the Uranian irregular satellites may shed light fault 2-by-2 binning, of 0.